1. Field of the Invention
The present invention relates to an electrically programmable ROM for example EPROM or EEPROM. These memories have the advantage that they may be programmed by the user. Moreover, in some cases, they may even be cleared then rewritten a certain number of times if need be. The data storage element thereof is a floating gate transistor. This transistor may have two states. In a first state, no charge is trapped on the floating gate.. A conduction channel may be installed between source and drain of this transistor. This latter may then conduct; it behaves like a closed switch. In a second state, electrons have been trapped on the floating gate. They prevent the creation of a conduction channel in the substrate between source and drain. The transistor is disabled and behaves like an open switch. The advantage of this type of storage element resides in the non volatility of the recorded data. The electric charges trapped in the floating gate only leak away slowly. The loss of charge in the floating gate determines the retention time of a memory point. At the end of this time, the stored data is no longer readable. This retention time is generally of the order of 5 to 10 years, it depends on the amplitude and on the duration of the voltage applied during programming. Typical programming values are 21 volts and 50 milliseconds.
The enabled or disabled state of the transistor is measured by feeding a selection pulse to its control gate. In the enabled state, no charge is trapped on the floating gate and the selection voltage applied causes saturation of the transistor. From the practical point of view, the tranistor is connected by a first main electrode to a bit line, voltage-biased by a generator; by its other main electrode it is connected to ground. The bit line is also connected to a current sensor: this sensor measures the current delivered into the line by the generator. When the transistor is enabled, it short circuits the generator and the sensor detects a current drop. This current drop is used subsequently as representative of the data corresponding to the programming state of the transistor concerned. In a second case, when the memory point is programmed, charges are trapped in the floating gate of the transistor. The selection voltage applied to the control gate is of opposite direction to the potential barrier created, in the conduction channel, by the charges stored in the floating gate. But it is then insufficient to modify the conduction of this channel: the transistor remains disabled. Consequently, the sensor at the end of the bit line does not perceive a current variation. At the time of selection of the memory point considered, it thus detects a state which is the reverse of the first case.
One of the most accessible fields in the technology of electrically programmable ROM's is that of memory cards. A memory card is a card, for example in the format of a banking credit card: an electronic integrated circuit is set in this card, and electric connection terminals are accessible on the surface so that operations may be carried out with the card in a terminal unit.
The fundamental problem of memory cards is that of frauds. Endeavors are made to prevent holders from falsifying the information contained in the memories of the cards. A first risk to avoid is that of the introduction, under poor conditions, of crictical information in the memory. Thus, in a first step, this information is introduced through series of electric pulses applied to the terminals of the memory. To be sure that the information has been correctly introduced, the contents of the memory is then checked by applying electric checking pulses to the access terminals.
2. Description of the Prior Art
But the effective retention power of the information stored in the memory points cannot be checked. Frauders are in fact suspected of tampering with the access terminals, for example by covering them with a minute layer of graphite which forms a resistance. Thus, programming of the data in the memory cannot be provided at a sufficient level for the retention of the information to correspond to a desired duration. It might be thought that such a fraud may be detected by feeding adequate electric pulses to the access checking terminal during checking. But frauders are also suspected of also tampering with the access checking terminals (probably in the same way). In other words, it is thought that a clever frauder could modify the programming conditions of the memory even if such programming is provided by the organization issuing the memory cards in question. Checking would then prove just as illusory since it would be practiced under the same conditions.
The present invention proposes an efficient solution to this problem: the potentials of the pulses used during checking are not, in the invention, potentials applied externally to the memory, but rather potentials produced therein. Fixing the useful potential level during programming irremediably determines the retention time of the information which it contains. With the potential level used by the checking pulses fixed intrinsically, the reality of the information stored in the memory may be checked.
This further means that, in accordance with the invention, if the information is programmed and if the programmed information is checked, it is done well or it is not done at all. In the state of the technique criticized, information could be programmed, then badly checked, so that for some time there is a false semblance of integrity of the information stored. After a certain time, the information would become illegible: in fact the ones would becomes zeros. This would singularly modify the information stored, especially if it was supposed to represent a bank account balance.